You can't tell without more context which of the possible interpretations of the continuous aspect was intended. But it doesn't communicate that she was working there only for July - nor does your last example with the simple past.
She was working at MacDonalds during July
suggests that it was probably only July she worked there, but it doesn't have to mean that. To say that unequivocally I would probably say
She worked at MacDonalds for the month of July.
(for some reason "for July" doesn't sound right to me). Note that, without context, I find I prefer the simple past there, so I am treating her working as a complete event. But if I were going on to talk about something that happened during July, I might well say
She was working at MacDonalds for the month of July.
Doing so places the temporal focus within the month, so what happened at the end of the month was still in the future, so to some degree this is talking about her (and their) intention, whereas with the simple past it is looking back at it complete.